The present invention relates to an electromagnetic solenoid drive apparatus for use in a vehicle for selectively turning on and driving an electromagnetic solenoid actuated by a battery as a power source.
In an automobile, for example, an electromagnetic solenoid is often used as a drive source of a so-called shift lock device or auto-door lock device, etc. In the shift lock device, for example, the electromagnetic solenoid is turned on only when an ignition key is turned on and a foot brake is operated in a state in which a shift lever for an automatic speed change gear is in a parking position.
The shift lock device also has a shift lock mechanism for locking or constraining the shift lever to be shifted from the parking position to another position in a state in which the electromagnetic solenoid is turned off. The shift lock device then additionally comprises a key interlock function to locate the shift lever reliably at the parking position when the ignition key is extracted from the device. In this case, the shift lock device further comprises an auxiliary electromagnetic solenoid turned on when the shift lever for the automatic speed change gear is located in a position except for the parking position, and a key lock mechanism for constraining the rotation of the ignition key to a LOCK position as an extractable position in a state in which the auxiliary electromagnetic solenoid is turned on. The electric power to the electromagnetic solenoid and to the auxiliary electromagnetic solenoid mentioned above is obtained form the battery mounted on the vehicle.
An output voltage of the battery is not constant at any time, but is changed depending on the charge state thereof, the strength of a load electic current and the like. Under this condition, since the power source of the electromagnetic solenoid is directly supplied from the battery through switches, etc., a drive voltage applied to the electromagnetic solenoid would accordingly be changed to a small or large value. As a result, in particular, when the output voltage of the battery is high, the drive voltage of the electromagnetic solenoid is also high so that a great operating noise is generated, which is uncomfortable in hearing. Further, the load electric current of the electromagnetic solenoid and thereby the heating amount thereof become undesirably large.
In order to solve such problems, in the conventional apparatus, it is considered that the drive voltage of the electromagnetic solenoid is restrained such that this drive voltage is less than a constant voltage by so-called a chopper control. However, in such a control, the structure of circuits becomes complicated, and a noise having a frequency according to a chopper control frequency is generted when the electromagnetic solenoid is held in a drive state.